In recent years, as a method for forming solder resist layers on printed-wiring boards for consumer use and industrial use, a method using a liquid solder resist composition that has excellent resolution, size precision, and the like, and can be developed has been widely adopted, instead of a screen printing method, in order to increase the density of wiring on the printed-wiring board.
Also, in recent years, optical elements such as light-emitting diodes are often mounted directly on a printed-wiring board on which a solder resist layer is formed to cover the printed-wiring board, for uses in: backlights of liquid crystal displays of mobile terminals, personal computers, televisions, and the like; light sources of illumination devices; and the like. Furthermore, as a result of the solder resist layer of the printed-wiring board on which optical elements are mounted containing titanium dioxide, the solder resist layer is whitened, which allows light emitted from light-emitting elements to efficiently reflect off the solder resist layer (see JP2012-78414A).
However, if the liquid solder resist composition contains titanium dioxide, when this liquid solder resist composition is exposed to light so as to be cured, the contained titanium dioxide may reflect or absorb light and possibly interfere curing of the liquid solder resist composition. In particular, if the liquid solder resist composition contains a large amount of titanium dioxide, it is difficult for a lower portion of the solder resist layer made of the liquid solder resist composition to be cured. If the lower portion of the solder resist layer is not sufficiently cured, poor quality is likely to occur, such as deterioration of resolution of the developed solder resist layer, occurrence of wrinkles in the solder resist layer due to a difference in shrinkage during curing between the lower portion and upper portion of the solder resist layer, and cracks due to the partial concentration of stress resulting from a difference in thermal expansion coefficients between the lower portion and the upper portion when the solder resist layer is heated.
Also, good storage stability is required for the liquid solder resist composition, and a low tackiness of the coating film made of the liquid solder resist composition is also required.